1. Field of the Invention
This invention relates to a method and a system for increasing fuel gas pressure delivered to the fuel system of a gas turbine engine.
2. Description of Prior Art
In many installations of stationary gas turbine engines which operate on natural gas fuel, an increase in pressure of the gaseous fuel is necessary since local natural gas supplies deliver the natural gas at insufficient pressures. Thus, a compressor is normally used to raise the pressure from the natural gas supply to an increased pressure required by the gas turbine engine. Conventional positive displacement gas compressors of the reciprocating or rotary screw types are used to accomplish such pressure increase. Such conventional compressors are typically standard units that are manufactured for other applications, and are driven by conventional electric motors. The reciprocating type of positive displacement gas compressor is more efficient than the rotary type but typically requires a space which is as large as the gas turbine engine and a heavy or substantial base, which results in higher installation and maintenance costs.
A centrifugal or axial compressor is normally more compact than a positive displacement compressor. However, with the combination of relatively low flow rates and relatively high pressure ratios required by gas turbine engines, particularly those with less than 10 MW power ratings, such flow parameters are outside of typical output ranges for conventional centrifugal or axial compressors. Thus, a centrifugal or axial compressor would normally require more than four stages, or operate at relatively high rotational speeds of about 50,000 rpm or more. Using four stages would likely preclude the possibility of reducing the cost of the compressor to below the cost of a comparable positive displacement compressor.
The power input of conventional fuel gas booster systems is in the range of 2-5% of the total output power of the gas turbine engine. Thus, even for a gas turbine engine having as little as a 1 MW output power rating, at least 20 kW would be required to power the conventional fuel gas booster. Electric motors of this size and larger are limited as drive units for the compressors, due to the relatively high shaft speed of the centrifugal or axial compressor. Expensive and cumbersome gear boxes would be necessary to match an electric motor drive unit with a centrifugal or axial flow compressor. Thus, it is apparent that there is a need for a relatively small turbocompressor or turbocharger system for boosting the pressure of gaseous fuel supplied to the fuel system of a gas turbine engine.